nir/lower_int64: do not try to clamp floats to int-range
The clamping isn't correct, because the exact values ended up getting rounded off a bit when converting back to floats.
But, converting floats to integers have undefined results when the float value doesn't fit in the integer. So let's not try to clamp the value here.
This was caught by digging at a Clang warning, see this thread for details: